Wide bandgap semiconductors like cadmium zinc telluride (CZT) have progressed in the last 20 years to become a promising material for gamma radiation spectrometers. The continued interest in CZT is attributed to its large absorption cross-section for gamma radiation due to high atomic number (stopping power) as well as its large bandgap that permits room temperature operation. The internal electric field in CZT crystals is highly dependent on the carrier and trap concentrations affecting both electron and hole mobility lifetime products (pr) which require a free path. The approximate bandgap of CdZnTe (with 10% Zn) is 1.6 eV which allows specific operation at room temperature without significant dark current and prevents excessive thermal generation of charge carriers. The ability to maintain a uniform electric field inside a CZT device is critical to optimal charge collection. If the actual distribution of the electric field differs significantly from that which is anticipated, the charge carriers may not flow as expected or may become trapped in vacancies or traps within the crystal.
Recent research into the uniformity of the internal electric field of single crystal CdZnTe and CdTe has been focused primarily on the infrared (IR) transmission data based on the Pockels effect. The Pockels effect is only observed in isotropic crystal structures, i.e., zinc-blended structures, due to their strong linear electro-optical coefficient. Some CZT crystals naturally exhibit a birefringence due to local defects and internal stresses that form during growth. However, the Pockels effect enhances the natural birefringence inherent in most materials allowing visualization of the internal electric field. The internal stress can be subtracted from the Pockels data by taking the difference between the transmission through the crystal with and without applied bias. Therefore, this technique is commonly used to determine the internal electric field of any CZT material while under bias to illustrate the electric field distribution in real time. This is accomplished by monitoring the changes in the electric field. Using this technique, a change in the electric field may be observed in response to external manipulations.
Examples of manipulations that can be performed on a crystal include exposure to light, magnetic fields, or physical stresses that have been applied to the surface. The efficiency of the carrier transport properties in CZT crystals is of great interest for the development of CZT based devices. In general, CZT materials typically exhibit hole transport mobility that is lower than electron transport mobility. However, in many cases, trapped charges in the crystal can affect both the hole and electron transport by as much as a factor of 10. By eliminating trapped charges in the low energy regime, both hole and electron transport efficiency can be increased throughout the entire volume of the crystal. In addition, an increase in the collection efficiency produces a higher signal to noise ratio. In the high energy regime, gamma sources produce excitation events in the entire bulk of the crystal. These events span the bulk of the crystal allowing higher collection volumes. The ability to eliminate carrier traps in the bulk would increase the overall charge transport in the crystal and also result in a change in the internal electric field of the crystal. This behavior could have a beneficial effect to several CZT utilized applications. Control of the internal electric field has previously been achieved using temperature to distort the internal carrier density near the cathode; however cryogenic temperatures were required to accomplish this feat. This also eliminates the advantage that CZT offers for room temperature operability.
Accordingly, there remains room for improvement and variation within the art.